Story: Landslides

Page 4 – People and landslides

Protective measures

As New Zealand’s population grows and spreads, producing
more buildings and road networks, the risk from landslides
increases. This is addressed by legislation that controls
resource development (Resource Management Act 1991), building
activity (Building Act 1991), and emergency management (Civil
Defence and Emergency Management Act 2000). The insurance
scheme administered by the Earthquake Commission also helps
to spread and mitigate the risk. Soil conservation and
forestry techniques are employed to reduce landslides on
productive land.

Threats to property

In situations where buildings and transport networks are
threatened, engineers, geologists and other earth scientists
investigate and then set up methods for controlling landslide
activity and reducing the risk to property. Developers of new
subdivisions on hillsides are often required to hire experts
to assess the stability of slopes and the likelihood of
landslides.

Landslides and the Clyde dam

Landslides can be incredibly costly. In the 1980s, when a
hydroelectric dam was being built on the Clutha River in
Central Otago, there was a budget blow-out. Engineers were
very worried about landslides above the Cromwell Gorge,
including the particularly troublesome Cairnmuir landslide.
When the dam was raised they feared that the groundwater
would also rise and that huge, slow-moving landslides might
become active. If one of these ploughed into the newly
created Lake Dunstan it could produce a wave that overtopped
the dam.

To be on the safe side, the slips were stabilised by
drilling tunnels to drain the water, and by reinforcing the
toe of the slides. The surface of the Cairnmuir landslide was
paved and terraced to seal out water, creating a bizarre
amphitheatre in the hills above the lake.

Investigating and stabilising the landslides cost $936
million (2005 value).

Road warnings

Large orange and yellow road signs depicting crumbling
hillsides will be familiar to many motorists. Before New
Zealand adopted symbolic international road signs in 1987,
roadside warnings of landslides read ‘Slips’ or ‘Falling
debris’, which often confused tourists.

Each year local councils, roading authorities, private
landowners and railway operators spend millions of dollars
clearing slips from roads and railway lines. It is a
never-ending task – there are always more floods or
earthquakes to come, and there is plenty of rock and soil
waiting to tumble down.

Unlucky hunters

In August 1972 two hunters were returning from a week
near the Clyde River in Canterbury’s back country. The
weather had been rough with rain and snow, and the Clyde
was running high. A rock slide engulfed them as they were
traversing a moraine bank 25 metres high and 100 metres
long. The sliding rocks killed them and carried their
bodies into the river.

Threat to life

Landslides often kill people in combination with other
natural hazards such as earthquakes and floods. High rainfall
and earthquakes often trigger landslides. The Murchison
earthquake of 1929 killed 17 people – 16 as a result of
landslides it generated. One estimate attributes 362 deaths
to landslide-related causes between the 1840s and the early
2000s.

In general, landslides are more common in New Zealand than
many countries because of the terrain and less stable
conditions. However, landslides cause few deaths in New
Zealand because there are few settlements in mountainous
terrain and the population density is relatively low.